Photolithography is a process to transfer a pattern from a photomask to a substrate by means of exposure. During this process, the substrate is supported on a workpiece stage, and an exposure device incorporated in a photolithography machine projects an image of the photomask pattern onto the surface of the substrate. Photolithography process performed by the photolithography machine is critical to the fabrication of semiconductor devices. In the photolithography machine, the workpiece stage is operative to move under a projection objective while carrying the substrate or wafer in coordination with a mask stage so that the substrate or wafer is exposed by light from the projection objective as desired. Therefore, control accuracy of the workpiece stage directly determines how well the photolithography process can be performed, so it is necessary for the workpiece stage to have good control accuracy. When the workpiece stage is moving, it will exert a reaction force directly on an inner frame of the photolithography machine. This shock may lead to augmented vibration of the inner frame and various components carried thereby, which can harm or even disable the exposure process.
FIG. 1 shows a conventional exposure device including an illumination system 11, a mask stage 12, a projection optical system 13 and a workpiece stage 20. The workpiece stage 20 includes two Y-direction motors that are disposed in parallel on an outer frame. An intermediate guide rail 21 connects, at its two ends, movers of the Y-direction motors so that it is perpendicular to both of the Y-direction motors and forms an H-shaped assembly therewith. Actuated by the Y-direction motors, the workpiece stage 20 can travel a long distance in the Y direction. An X-direction motor 22 is disposed on the intermediate guide rail 21. The X-direction motor 22 has a mover supporting a mounting plate 34 on which a coarse-motion stage 24 is arranged. The coarse-motion stage 24 is slidably connected to the intermediate guide rail 21 via an X-slider 23. The X-direction motor 22 is operable to drive the coarse-motion stage 24 to move in the X direction. A fine-motion stage 25 is provided above the coarse-motion stage 24 in a non-contact manner. The fine-motion stage 25 is finely tunable by a plurality of voice coil motors 33 which are disposed between the coarse-motion stage 24 and the fine-motion stage 25. Under the fine-motion stage 25, reinforcing members 26 are attached to opposing edges thereof, and air bearings 27 are provided under the respective reinforcing members 26. Correspondingly, there are two step guides 28 on the two sides of the intermediate guide rail 21, which are spaced apart from each other by the same distance. The step guides 28 provide surfaces on which the air bearings 27 can create air films for supporting the fine-motion stage 25 at the opposing edges. The step guides 28 are in mechanical connection with the intermediate guide rail 21 via connecting mechanism 29 and in slidable connection with the inner frame 31 via a Y-slider 30. A substrate holder 32 is attached to the fine-motion stage 25, and the substrate P is retained on the substrate holder 32.
As the intermediate guide rail 21 is disposed on the Y-direction motors in this conventional device, the Y-direction motors have to directly bear the significant load imposed by all the components on the intermediate guide rail 21 as well as by the step guides 28. As a result, the workpiece stage exhibits poor modal and vibration characteristics. In addition, the step guides 28 in slidable connection with the inner frame 31 via the slider may pose vibration-causing impacts on the inner frame 31, and friction that may occur in the slidable connection can affect the control accuracy.